This invention relates to a system for the separation of a gas mixture, especially a mixture chiefly of hydrocarbons, and particularly where the gas mixture is subjected to partial condensation, and the resultant liquid is subjected to rectification.
Such a system is used, for example, in the production of ethylene from the gases produced in a plant for the cracking of hydrocarbons. Usually, the cracked gas is compressed and then partly condenced by single or multistage cooling. The fractions thus obtained are conventionally fed into a rectifying column, in which column a sharp separation into two fractions occurs, the first of which contains C.sub.2 -hydrocarbons and components having a lower boiling point, and the second C.sub.3 -hydrocarbons and components having a higher boiling point. In order to condense the lower-boiling fraction at the head of the rectifying column, a large amount of cooling to relatively low temperature levels is required, while steam is utilized as reboiler heat at the bottom. Thus, this known method entails the use of a large amount of power.
From DE-OS 26 08 404 [Published Sept. 8, 1977], a method is already known for the separation of gas where power requirements are reduced by dividing the rectification between two separate columns which operate in different temperature ranges and at relatively low differences in temperature between the head and the bottom. An English language equivalent of DE-OS 26 08 404 is U.S. patent application Ser. No. 773,377 of Koening, now abandoned.